Some of the Matrix folks are active on HN, maybe they mentioned something in the discussions back then? But I wouldn't be surprised if it wasn't much if anything - my impression was that Gitlab didn't really know what to do with gitter, so giving it to messaging-focused project would make some sense.

No, bridged rooms generally are reachable from any instance. It's just that the bridge talks to matrix.org and matrix.org "hosts" (that's not 100% accurate afaik to how Matrix works, but close enough for this) the room just like any other matrix.org room, that is also reachable from other instances.